fix(sdk): address PR #2100 review feedback on attachSync

- Script-mirror filter changed from an exact "/script/gsap" match to
  path.startsWith("/script/") — the documented contract is "never mirror
  script-tag rewrites," not just today's one known path; startsWith covers
  any future script-kind patch under the same intent.
- _syncDetach is now cleared when the caller invokes the returned detach
  function directly, not only on the next attachSync call — avoids holding
  a stale (already-unsubscribed) reference between an explicit detach() and
  a later attachSync(other).
- The initial applyOverrideSet call is now wrapped in try/catch: a bad
  initial snapshot no longer prevents the ongoing patch subscription from
  attaching, matching the SDK's existing swallow-and-warn precedent for
  silent-failure paths (adapters/iframe.ts's tainted-canvas warning).
- Added a test proving declareVariable/removeVariable (the /variable-decls/
  patches PR #2098 introduces) mirror onto the live document's
  data-composition-variables attribute — the existing suite only covered
  setVariableValue's CSS-custom-property path, not the schema-metadata path.

packages/sdk/src/adapters/iframe.sync.test.ts

@@ -219,6 +219,21 @@ window.__timelines = { t: tl };</script>
     expect(liveRoot.style.getPropertyValue("--accent")).toBe("#0f0");
   });
 
+  it("mirrors declareVariable/removeVariable onto the live document's schema attribute", async () => {
+    const iframe = mountIframe(BASE_HTML); // no data-composition-variables at all
+    const comp = await openComposition(BASE_HTML);
+    const adapter = createIframePreviewAdapter(iframe);
+    adapter.attachSync(comp);
+
+    comp.declareVariable({ id: "accent", type: "string", label: "Accent", default: "#fff" });
+
+    const liveDocEl = iframe.contentDocument!.documentElement;
+    expect(liveDocEl.getAttribute("data-composition-variables")).toContain("accent");
+
+    comp.removeVariable("accent");
+    expect(liveDocEl.getAttribute("data-composition-variables")).not.toContain("accent");
+  });
+
   it("mirrors setTiming onto the live element's data-start/data-end attributes", async () => {
     const iframe = mountIframe(BASE_HTML);
     const comp = await openComposition(BASE_HTML);